1. Field of the Invention
The present invention relates pyroelectric power generation systems, and more particularly to pyroelectric systems to harvest thermal energy from waste heat.
2. Description of the Related Art
Solar, thermal, and mechanical energy (including vibration, motion, and wind) are three major renewable energy sources in the environment that are not fully utilized. For example, some studies estimate that fifty percent of the energy generated annually from all sources is lost as waste heat. Specifically, the current energy efficiency of automobile engines is only about 33% for mechanical drives. The remaining 66% of energy is wasted as thermal recuperation including engine heat up, engine coolant/oil heat exchange, and exhaust gas, in which only 34% of total energy is used for driving a vehicle. Efforts to increase engine efficiencies include new combustion engine designs and fuel modification, but high waste of heat energy remains an issue, and a potential source of renewable energy.
Silicon based solar cells that dominate this field typically harvest 15-20% of available solar energy, with the balance going to waste thermal energy. While several efforts to increase efficiency utilizing heterojunction photovotalics, like silicon germanium or dye-sensitized solar cells have been published, the efficiency or stability is still very low, although some higher efficiencies are beginning to be realized for solar cells in laboratory tests. Similarly, a majority of piezoelectric and electromagnetic vibration/motion harvesting technologies harvest less than 10% of mechanical energy inputs under ideal laboratory conditions.
Thermal energy harvesting research is largely focused on thermoelectrics, for which the large P-N junction temperature gradient required for effective use of thermoelectrics necessitates complex device designs and yields very low efficiencies. Examples of some approaches that have been developed recently include attempts to utilize waste heat from exhaust gas generated by engines using thermoelectric devices. The thermoelectric elements (p-type and n-type) are connected to harvest thermal energy from exhaust gas flow to generate electrical energy. Fuel economy of these system increased by only 3-5%, due to the low efficiency (5%) of the thermoelectric element. Additionally, in many thermoelectric systems, a cooling system is embedded to create a temperature difference (ΔT) between hot side (exhaust gas flow) and cold side (coolant flow). This results in a complicated system and weight penalty. In addition, to generate significant amount of electricity, engines have to be running, and get hot enough to reach the specific temperature difference required for a thermoelectric device to function.
Pyroelectricity is the ability of certain materials to generate surface charge and build a temporary voltage when they are heated or cooled. As opposed to thermoelectric materials which require a temperature gradient between two portions of the thermoelectric material to generate a charge, pyroelectric materials generate a charge and build a temporary voltage when the pyroelectric material itself increases or decreases in temperature. Thus, no need to maintain a temperature gradient between portions of a pyroelectric material is required to generate a charge and build a temporary voltage. A pyroelectric material can be repeatedly heated and cooled (analogous to a heat engine) to generate usable electrical power. It is calculated that a pyroelectric material in an Ericsson cycle could reach from 50% to 84-92% of Carnot efficiency. These efficiency values are for the pyroelectric material itself, and ignores losses from heating and cooling the substrate, other heat-transfer losses, and all other losses elsewhere in the system.
In the last half century, several pyroelectric energy harvesters have been invented to convert waste thermal energy into electrical energy. Compared with thermoelectric energy harvesters, where large P-N junction temperature gradients required for effective use of thermoelectrics necessitate complex device designs and yield very low efficiencies, a pyroelectric energy harvester can be designed more simply and with higher efficiency. Pyroelectrics have advantages over thermoelectrics since thermal gradients on the order of those required for thermoelectrics are not needed for voltage generation. However, previous pyroelectric energy harvesters have not been commercialized.
Massive thermal cycle energies exist on the surface of the earth. For instance, 1) an average of 10° C. difference exists between daytime and nighttime on Earth's surface, 2) more than 50° C. difference is realized when an isolated object is exposed to direct sunlight, and 3) more than 100° C. thermal variation is generated by engine exhaust manifolds during a standard drive cycle. Further, large amounts of waste heat are generated from refrigeration and heat pumps. This can account for over 50% of terrestrial energy consumption.
Massive thermal cycle energies exist in space environments, as well, and, compared to on the earth, a larger magnitude of thermal cycles is available in space environments (e.g., temperature ranges from −59° C. to 93° C. (ΔT=152K) per daily cycle at low Earth orbit, and from −90° to 150° C. (ΔT=240K) on the Lunar surface).
A practical approach to harvest the massive thermal cycle energy as a renewable power source is needed as part of the renewable energy technologies that need to be developed.